Methods of lithographically producing spatial fine structures comprising the steps of forming a spatial fine structure of a material, and microscopically examining whether the desired fine structure is present, are used for forming conductor fine structures or insulator fine structures in the production of electronic semiconductor devices. To this end, for example, a layer of a radiation-sensitive material is applied to a semiconductor substrate. The term “radiation-sensitive” refers to the fact that the state of this material can be changed by means of radiation in such a way, for example, that it is possible to easily remove the irradiated areas of the layer with a solvent, while the not irradiated areas of the layer remain untouched by the solvent. Typically, various phenolic resins, acrylic polymers, alicyclic polymers and fluoric polymers are used as a basis for suitable radiation-sensitive materials. Any radiation-sensitive material is particularly sensitive to an electromagnetic radiation of a certain wavelength range, which may vary from the medium UV-range to the X-ray (Roentgen) range. The radiation-sensitivity of the material of the layer is, for example, based on photo-induced local conversion into an acid, or, complementary, on photo-induced local cross-linking. The term “radiation-sensitive” can, however, also refer to a sensitivity of the material to electron radiation or to other particle beams. In each case of the known method of lithographically producing spatial fine structures, the layer applied to the substrate is irradiated within defined spatial areas in order to convert the material in these spatial areas as compared to the not irradiated areas so that these areas of the material are distinguishable from each other. Subsequently, developing of the illuminated layer takes place, which—as a rule—removes the material in those spatially defined areas, in which the layer has been irradiated. In principle, however, developing can also result in that the material of the layer is removed in the not irradiated areas. In those methods known from the production of electronic semiconductor devices the presence of the desired fine structure is checked by electron microscopy. Electron microscopy is used in order to spatially dissolve the obtained fine structure to a maximum extent. The efforts to be taken for a scanning electron microscopic examination of the layer, however, are considerably high. The substrate with the respective layer must be locked into high vacuum equipment, as electron microscopy can only be operated under high vacuum. This means that no volatile substances, which could impair the high vacuum and/or damage the high vacuum equipment, may be emitted by the substrate, the layer, and any substance or article which is brought into the high vacuum equipment together with the substrate and the layer. Further, the efforts for the installation and the operation of an electron microscope are quite high.
At present, however, very few alternatives to an electron microscopic investigation are available in methods of producing lithographic fine structures, if a resolution is to be achieved within the range of better than 150 nm. State of the art circuit board tracks in microelectronics already display widths down to 90 nm with even narrower track distances. The available alternatives to electron microscopy are methods in which the fine structure to be examined is scanned with a probe. Atomic Force Microscopy (AFM) and Scanning Nearfield Optical Microscopy (SNOM) belong to those alternatives, which require no high vacuum, but which, like in the case of raster electron microscopy, require an accurate and thus laborious adjustment of the cleaned fine structure with regard to the sensible arrangement for displacing the respective probe, and which are extremely slow as compared to the size of the examined fine structure. Thus it seems to be that the efforts for microscopic investigation in the known methods of lithographically producing spatial fine structures can only be significantly reduced in that not all lithographically produced fine structures, but only a few samples of them are examined. This, however, quite substantially increases the danger of incorrectly produced electrical devices.
A method of producing spatial fine structures comprising the steps of adding a luminophore to a material, forming a spatial fine structure of the material, and fluorescence-microscopically examining whether the desired fine structure is present, luminescence light emitted by the luminophore being measured, is known from US 2003/0036006 A1. Here, a so-called photoresist is doped with a luminophore which is tuned to the photoresist in such a way that it exhibits a different fluorescence behavior, i.e. another wavelength of the fluorescence light, depending on whether it is present within an irradiated or a not irradiated area of the photoresist. The exposed photoresist is analyzed using conventional fluorescence microscopy by which a spatial resolution in the order of magnitude of the wavelength of the fluorescence light is achieved. Further, the fine structure which is generated in the photoresist by irradiation is not imaged directly in the known method; instead, it is determined along a line across the photoresist whether the fluorescence light displays a certain intensity modulation having the spatial frequency of the desired fine structure. If this intensity modulation is present, it is assumed that an illumination device for illuminating the photoresist is correctly focused. The maximum intensity modulation of less than 10%, which is, for example, registered with a fine structure of lines having a distance of 440 nm using fluorescence light having a wavelength of about 515 nm, is completely insufficient for actually imaging the fine structure, which was produced by illuminating the photoresist. Line distances of fine structures produced in state of the art semiconductor technology by means of photoresists are smaller than 100 nm and could in no way be detected by the fluorescence-microscopic method described in US 2003/0036006 A1. According to US 2003/0036006 A1 itself, an electron microscopic imaging method (SEM) is used for actually imaging the produced fine structure.
A method of fluorescence-microscopically examining a sample, which and/or an interesting structure of which has previously been stained with a fluorescence dye, is, for example, known from DE 101 54 699 A1, corresponding to U.S. Pat. No. 7,253,893 B2. In this method, which is also known as STED (Stimulated Emission Depletion) fluorescence microscopy, a fluorescence dye, with which the sample and/or the interesting structure of the sample has been stained in a previous step, is first transferred into an excited energetic state with an exciting optical signal. The usual diffraction limit of optical methods of λ/(2n sin α) applies to the spatial resolution In this optical excitation, λ being the wavelength of the light used, n being the refractive index of the sample, and a being the half aperture angle of the objective used. In order to get below this limit, the optically excited state of the fluorescence dye is de-excited again with a de-exciting optical signal outside a desired measuring point, in which the de-exciting optical signal displays a zero point; i.e. the fluorescence dye in the sample is forced to stimulated emission by means of the de-exciting optical signal everywhere outside the measuring point. The dimensions of the resulting still fluorescent measuring point, i.e. the spatial resolution of remaining fluorescence, can be reduced clearly below the usual optical resolution limit in that the de-exciting optical signal is applied to the sample at such an intensity outside the desired measuring point that a saturation is reached in the de-excitation by stimulated emission. Thus, the fluorescence dye in the sample is still in the excited state only in a very closely limited area around the zero point of the intensity distribution of the de-exciting optical signal. Accordingly, it can only fluoresce within this spatially limited area.
According to Hell, Nature Biotechn., 21, 1347-1355, the size of the fluorescent measuring point Δx and thus the spatial resolution in STED fluorescence microscopy follows Δx=λ/(2n sin α√(I/IS)), λ being the wavelength of the de-exciting optical signal, n being the refractive index of the sample, α being the half aperture angle of the objective used, I being the irradiation intensity of the de-exciting optical signal and IS being a saturation intensity. The saturation intensity IS is a characteristic intensity, at which, from a statistics point of view, the fluorescence dye in the sample is de-excited by 50% due to the effect of the de-exciting optical signal.
Another method of fluorescence microscopy in which the spatial resolution can fall below the diffraction limit is known as GSD (Ground State Depletion) fluorescence microscopy. Here, the fluorescence dye is not de-excited after it has initially been excited in order to transfer it into a non-fluorescent state outside the desired measurement point. Instead, the fluorescence dye outside the desired measuring point is transferred out of its ground state into a state in which it cannot be excited for fluorescence even before the excitation of the fluorescence dye takes place.